What looks like a very nice circuit is available at Harry's site.


http://w1.859.telia.com/~u85920178/use/gdo.htm

This site includes all the info you would need to contruct your own
dip meter, including coil winding information.

Please forgive me if this has been noted before; this was the first
site to come up on Google with the request of "Grid Dip Meter
Circuit."

73 de WA2AFD